In the production of textile fabrics, the warp threads and the weft threads intersect precisely at right angles. However, during subsequent processing of the fabrics, the fabric may become warped. In the production of knitted fabrics on circular knitting machines, the resulting circular fabric is cut and the knitted fabric is generally diagonally warped after cutting. In both cases, the warping is corrected by straightening machines which require the draft angle as a control value. Therefore, it is necessary to determine the draft angle of the weft threads on a moving textile fabric so that the angle can be corrected if required.
German Patent No. 16 35 266 discloses an apparatus for measuring the draft angle wherein a single aperture having a photosensor situated behind it is oscillated by an electrodynamic driving system, the oscillating movement taking place about a central axis at about the mechanical resonant frequency of the system. The speed of the oscillating movement, therefore, is predetermined by the system. The output signal of the photosensor is summed up by an amplifier, the sign of the amplification being always reversed when the draft angle exceeds the central angle. Therefore, the signal summed up over one period becomes zero when the measured value of the draft angle is symmetrically distributed about the central angle. This is the case when the weft thread has the same direction as the central angle. In addition, the known system provides a follow-up control device which, according to the measured value at a given moment, adjusts the whole system or the central angle in a manner such that the central angle always extends parallel with the weft thread. Thus, a direct measurement of the course of the weft thread or of the draft angle is possible by measuring the central angle.
This known system is deficient in that it requires mechanically moving parts which are necessarily subject to abrasion and wear. Because the speed of oscillation (resonant frequency) must be adapted to the advancing speed of the passing fabric, the inertial mass of the moving parts limits the speed at which the fabric can be advanced.
An apparatus is known wherein opposite to a light source, there are situated two photocells arranged behind slits whose central axes form angles with each other. From the differential signal of the photocells, a value for the angular course of the weft thread is determined without need for the system to be mechanically moved. What is important in this system is a correct measurement of the amount of light that penetrates through the textile sheet which, in the case of a single photosensor, causes certain difficulties, since the amount of light which penetrates through the fabric depends not only on the distances from each other of the weft threads and on the thickness thereof but also on the color of the textile sheet. In the case of printed materials and irregular textile fabrics, this causes difficulties. However, since in the known apparatus two electro-optical systems must be adjusted to each other, the difficulty associated with operation of the system is substantially increased. Another problem is the poor "pull-in range" of the system which is determined by the angle between the two slits.
Taking the above prior art as a point of departure, the problem solved by this invention is to perfect the process and apparatus as described above, in the sense of making it possible to obtain by simple, means a correct measurement of the course of the weft thread with low susceptibility to failure.
This problem is solved by the process and the apparatus of the invention.